BASIC TERMS IN VLSI ~ ECE

Thursday, 14 February 2013

BASIC TERMS IN VLSI

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1.Define an Integrated circuit.
An integrated circuit(IC) is a miniature ,low cost electronic circuit consisting of active and passive components fabricated together on a single crystal of silicon.The active components are transistors and diodes and passive components are resistors and capacitors
.
2.What are the basic processes involved in fabricating ICs using planar technology?
1.Silicon wafer (substrate) preparation
2.Epitaxial growth
3.Oxidation
4.Photolithography
5.Diffusion
6.Ion implantation
7.Isolation technique
8.Metallization
9.Assembly processing & packaging

3.List out the steps used in the preparation of Si – wafers.
1.Crystal growth &doping
2.Ingot trimming & grinding
3.Ingot slicing
4.Wafer policing & etching
5.Wafer cleaning

4. Write the basic chemical reaction in the epitaxial growth process of pure silicon.
The basic chemical reaction in the epitaxial growth process of pure silicon is the hydrogen reduction of silicon tetrachloride.
1200oC

SiCl₄ + 2H₂---------------> Si + 4 HCl

5. What are the two important properties of SiO2?
1.SiO2 is an extremely hard protective coatng & is unaffected by almost all reagents except by

hydrochloric acid. Thus it stands against any contamination.
2.By selective etching of SiO2 , diffusion of impurities through carefully defined windows in the
SiO2 can be accomplished to fabricate various components.

6. Explain the process of oxidation.
The silicon wafers are stacked up in a quartz boat & then inserted into quartz furnace tube. The Si wafers are raised to a high temperature in the range of 950 to 1150oC & at the same time, exposed to a gas containing O₂ or H₂O or both.The chemical action is

Si + 2H₂O --- > Si O₂+ 2H₂

7. What is meant by molecular beam epitaxy(MBE)?
In the molecular beam epitaxy, ilicon along with dopants is evaporated.The evaporated species are transported at a relatively high velocity in a vacuum to the substrate.The relatively low vapour pressure of silicon & the dopants ensures condensation on a low temperature substrate.Usually, silicon MBE is performed under ultra high vacuum (UHV) condition of 10-8 to 10-10 Torr.

8. What are the advantages of Molecular Beam Epitaxy( MBE )?
( i ) It is a low temperature process, useful for VLSI. This minimises outdiffusion & autodoping.
( ii ) It allows precise control of doping& permits complucated profiles to be generated.
( iii )Linear doping profile desirable for varactor diode in FM , can be obtained with MBE.
( iv )Wider choice of dopants can be used.

9. What are oxidation induced defects in semi conductor?
1.Stacking faults
2.Oxide isolation defects

Stacking faults:

Structural defects in the silicon lattice is called oxidation induced stacking faults.The growth of
stacking faults is a strong function of substrate orientation , conductivity type & defect nuclei present.The
stacking faults formation can be suppressed by the addition of HCl.

Oxide isolation defects :

The stress along the edges of an oxidised area produce severe damage in the silicon. Such
defects results in increased leakage in nearby devices.High temperatures (around 950oC ) will prevent stress induced defect formation.

10. What is bird’s beak?
In local oxidation process, the oxidation of silicon proceeds slightly under the nitride as well. Also, a large mismatch in the thermal expansion co-efficient of Si₃N₄& Silicon results in damage to the semi conductor during local oxidation.This damage can be greatly reduced by growing a thin layer of SiO2 prior to placement of theSi₃N₄mask.Typically 100 to 200Ao is used for this pu_rpose. Unfortunately, this greatly enhances the penetration of oxide under the nitride masked regions , resulting in oxide configurations called bird’s beak.

11. What is lithography?
Lithography is a process by which the pattern appearing on the mask is transferredto the wafer.It involves two steps: the first step requires applying a few drops of photoresist to the surface of the wafer & the second step is spinning the surface to get an even coating of the photo resist across the surface of the wafer.

12. What are the different types of lithography? What is optical lithography?
The different types of lithography are :
1.Photo lithography
2.Electron beam lithography
3.X ray beam lithography
4.Ion beam lithography

Optical lithography:

Optical lithography comprises the formation images with visible or UV radiation in aphoto resist using contact, proximity or projection printing.

13. What are the two processes involved in photolithography?
a) Making a photographic mask
b) Photo etching
The development of photographic mask involves the preparation of initial artwork and its reduction,decomposition of initial artwork or layout into several mask layers.
Photo etching is used for the removal of SiO2 from desired regions so that the desired impurities can be
diffused.

14. Distinguish between dry etching & wet etching.

15. What is meant by reactive plasma etching?
The term reactive plasma is meant to describe a discharge in which ionization & fragmentation of gases takes place& produce chemically active plasma species, frequently oxidizers and reducing agents.Such plasmas are reactive both in the gas phase & with solid surfaces exposed to them. When these interactions are used to form volatile products so that material is removed or etching of material form surfaces that are not masked to form lithographic patterns, the technique is known as reactive plasma etching.

16. What is isotropic & anisotropic etching processes?
Isotropic etching is a wet etching process which involves undercutting.
Anisotropic etching is a dry etching process which provides straight walled patterns.

17. Define diffusion.
The process of introducing impurities into selected regions of a silicon wafer is called diffusion. The rate at which various impurities diffuse into the silicon will be of the order of 1 micro meter per hour range of 900oC to 1100oC .The impurity atoms have the tendency to move from regions of higher concentrations to lower concentrations.

18. What is dielectric isolation?
In dielectric isolation, a layer of solid dielectric such as SiO2 or ruby completely surrounds each
components thereby producing isolation, both electrical & physical. This isolating dielectric layer is thick enough so that its associated capacitance is negligible. Also, it is possible to fabricate both pnp & npn transistors within the same silicon substrate.

19. What is metallization?
The process of producing a thin metal film layer that will serve to make interconnection of the various components on the chip is called metallization.

20. What are the advantages of ion implantation technique?
1. It is performed at low temperature. Therefore, previously diffused regions have a lesser tendency for lateral spreading.
2. In diffusion process, temperature has to be controlled over a large area inside the oven,whereas in ion implantation process, accelerating potential & beam content are dielectrically controlled from outside.